Solenoid controlled door locks, for example those used in prisons or other security installations are well known in the art. An exemplary solenoid controlled lock is disclosed in U.S. Pat. No. 4,593,543, the specification of which is incorporated by reference. This lock includes a solenoid having high and low current coils that are wound in series. In the deenergized state, the solenoid plunger is fully extended and a normally closed switch shunts the low current winding. Upon the initial application of power to the solenoid, only the high current coil conducts. As the plunger approaches its fully retracted or seated condition, the switch is tripped, allowing both the high and low current coils to conduct and exert sufficient force to fully retract the plunger and maintain it in a seated condition.
This type of lock has the general disadvantage of requiring two coils and a thermal protector to prevent damage to the high current coil in case the switch is not activated, e.g., because the plunger is stuck, the lock has been tampered with, or due to field installation problems.
In locks using single coil solenoids, the power consumption of the coil is a significant concern. One commercially available power supply (Schlage Electronics Model 700 Hardware Drive Module) periodically provides a short duration overvoltage pulse to the coil to ensure that the plunger is in its seated position. The plunger is maintained in that position by a reduced holding voltage that is applied between overvoltage pulses. The lower holding voltage reduces the solenoid current and thereby decreases the overall power consumption of the lock.
Other known devices, for example, the Synektron Model 10-G100C17 Controlled Field Actuator (CFA), employ a Hall-effect sensor to control the application of high frequency voltage pulses to the solenoid by varying the duty cycle of the pulses as a function of the plunger stroke. Because of the nonlinear solenoid spring force, a greater force is required to move the plunger from an extended position than is required to hold the plunger in its seated position. To achieve the force necessary to move the plunger from an extended position, the width of the high frequency pulses is greatly increased. Conversely, as the plunger nears its seated position, the CFA automatically shortens the pulse width.
The duty cycle of the CFA pulse train varies throughout the entire range of plunger motion, however, the pulse frequency is constant. Thus, the CFA is a relatively complex servomechanism capable of performing more sophisticated functions than the simple on/off function required for lock applications. Moreover, because the Hall-effect sensor requires a special solenoid with 6 rather than 2 wires, it is not cost efficient to use a CFA for lock applications.
It is therefore an object of this invention to provide a simple inexpensive solenoid controlled lock that can be operated at reduced power levels.